Implement the virtio block 'get-ident' operation. This eliminates the
annoying verbose boot error of the form
g_handleattr: vtbd0 bio_length 24 len 28 -> EFAULT
The ident returned by bhyve is a text string 'BHYVE-XXXX-XXXX', where
the X's are the first bytes of the md5 hash of the backing filename.
Approved by: re (gjb)